Plastic chips tap IR, solar power

PORTLAND, Ore.  Development of a low-cost plastic infrared photovoltaic material by a group at the University of Toronto could herald a major step forward for solar power, its creators believe, by enabling solar-powered systems to also harvest infrared emissions.

The material embeds various-size nanoparticles-or quantum dots-in a polymer suspension. "We have designed a plastic device that is physically flexible-you could even paint it onto things by putting it in a solution," said Toronto EE professor Ted Sargent. "However you deposit it, after drying you have a nice, thin, smooth film that provides the basis for an electronic device."

Sargent believes large-area plastic infrared photovoltaics could become a major marketplace within 10 years, depending on how low their cost goes. But they were not the original research target.

"Our first device was an infrared detector, which converts infrared optical signals into an electrical signal," said Sargent. "As a bonus, because we hadn't anticipated that this would work, we found that it was also a good photovoltaic material capable of harnessing the sun's power.

"There are already infrared photovoltaic cells that are not plastic, and there are already plastic photovoltaic materials," he went on. "What we have done for the first time is combine the two to create a plastic infrared photovoltaic material-that has not been done before."

Others have resorted to exotic technologies to scoop up the entire spectrum of energy from the sun. For instance, Sandia National Labs has created a Stirling-engine-based solar dish that it plans to use in 11-square-mile farms that generate as much electricity as the Hoover Dam.

But Sargent argues that his plastic photovoltaic material can be tuned, with almost any variety of embedded quantum dots, to whichever spectrum is required-both visible and infrared nanoparticles-for a full-spectrum solar cell.

"We think it is quite important," said Sargent. "In the past, photovoltaic cells have not harvested that other [infrared] half of the spectrum, but our device does for the first time."

Nanoparticles enhance a material's quantum-mechanical properties because their electrons are confined to a volume smaller than the electron's wavelength.

"In our materials the wavelength of an electron is about 20 nanometers," said Sargent. "But our nanoparticles-the quantum dots that we used-ranged from 2 to 6 nm in diameter. So we were very very strongly squeezing the electrons.

"The size of the nanoparticles determines the wavelength to which your device will be sensitive," he continued. "By making [semiconducting] particles that are only a few nanometers in size, we squeeze electrons down so far that their wavelength properties can no longer be ignored [in our calculations]-it becomes a quantum-mechanical phenomenon, a so-called quantum dot."

Sargent chose the 2- to 6-nm range of nanoparticle sizes in order to cover a nearly continuous band of wavelengths starting in the infrared and extending into the visible. However, he said, for the current demonstration he was just trying to achieve the "world's first," not the world's most efficient. Next, his group has to prove that its design can actually attain the kind of efficiency that would make it competitive with current silicon cells. The current 1-nm surface coating is, Sargent said, "too thick-we need to make it easier for the electrons to escape from the nanoparticles by making the coating thinner."

Sargent estimated the level of improvement he would consider high enough to make a difference in practical applications. Today, he said, plastic photovoltaic cells from other sources operate at about 6 percent efficiency. Sargent claimed that his design could have the potential to operate at upward of 30 percent efficiency, enabling it to compete with silicon cells.

"If we can improve efficiency by one or two orders of magnitude, then that will be major progress," he said.

Doctoral candidate Steve MacDonald performed most of the experiments at the University of Toronto, along with EE students Paul Cyr, Ethan Klem, Gerasimos Konstantatos, Larissa Lavina and Shiguo Zhang.

Funding came from the government of Ontario, the Natural Sciences and Engineering Research Council of Canada, Nortel Networks, the Canada Foundation for Innovation, the Ontario Innovation Trust, the Canada Research Chairs Programme and the Ontario Graduate Scholarship.